1. Field of the Invention
The present invention relates to catalytic reactor design. More specifically, the invention is a catalytic reactor for converting a fuel/oxidant mixture into a fuel/oxidant/product mixture and heat. The reactor employs an exothermic catalytic reaction channel cooled by numerous cooling channels, where the cooling fluid is a portion of the ultimate fuel/oxidant/product mixture. A further refinement in the invention incorporates geometric changes in the exothermic and/or cooling portions of the reactor to provide streamwise variation of the velocity of the fluids in the reactor.
2. Brief Description of the Related Art
Highly exothermic catalytic reactors with internal cooling are well known. While they have varying applications, the reactors are typified by exothermic reactions within the catalytic portion of the reactor and a cooling means to control the temperature within the catalytic portion to avoid a material failure, either of the substrate or the catalyst. Cooling in these reactors can be accomplished by a number of means, including placing the catalyst in a backside-cooled relationship with the cooling agent. A backside-cooling arrangement is particularly suitable for catalytic reactions that are both rapid and highly exothermic, such as catalytic combustion. In this arrangement, the catalyst substrate (typically a metal foil) is coated with an oxidation catalyst on only one side, the opposite side (or backside) remaining free of oxidation catalyst. The substrate is shaped and assembled, before or after catalyst coating, to create separate channels for exothermic reaction (in the channels coated with oxidation catalyst) or for cooling (in the channels free of oxidation catalyst). Fluid passing through the cooling channels removes a portion of the heat generated in the exothermic reaction channels.
An early example of a backside-cooled catalytic reactor for use in a catalytic combustion system is presented in U.S. Pat. No. 4,870,824 to Young et al. The e824 patent teaches the basic method of splitting a given fuel/air mixture flow into catalytic and non-catalytic passages. The '824 patent teaches the use of a ceramic substrate with multiple parallel channels, generally of the same shape and size, in which the walls which border and define each catalytic channel are coated with an oxidation catalyst on the sides facing the catalytic channel, but are not coated with an oxidation catalyst on the sides facing adjacent non-catalytic channels. By this method, the percentage of total reactants catalyzed in the reactor is no greater than the percentage of catalytic channels. The average temperature rise through the reactor is thus limited. In addition, the wall temperatures of catalytic channels bordering adjacent non-catalytic channels are controlled through the use of backside cooling.
Refinements of the basic structure taught by Young et al. are shown in U.S. Pat. Nos. 5,250,489 and 5,512,250 to Dalla Betta et al. In the '489 patent a metal substrate is used for improved heat conduction to the backside cooling fluid, and for greater resistance to thermal shock. Aluminum-containing steels are cited as being preferred. The '489 patent also teaches the use of non-similar shape and/or size channels, so that the flow split between catalytic and non-catalytic channels can be varied while retaining approximately half catalytic channels and half non-catalytic channels. Despite these changes, the fundamental structure claimed by Young et al., namely a multitude of catalytic channels and adjacent non-catalytic channels, is retained.
The '250 patent further refines the structure claimed by the '824 and '489 patents. In the '250 patent, Dalla Betta et al. teach a structure in which periodic alterations in channel shape provide different wall heat transfer rates in the catalytic channels and non-catalytic channels. Again, however, the fundamental structure claimed by Young et al., namely a multitude of catalytic channels and adjacent non-catalytic channels, is retained. Furthermore, while the '489 and '250 patents to Dalla Betta et al. teach catalytic and non-catalytic channels of different shape and tortuosity, the average channel properties (over some finite length) are not varied in the longitudinal direction, so that the catalytic reactors taught are effectively one- or two-dimensional in terms of channel flow properties such as bulk heat transfer coefficient, velocity, or average cross-sectional shape or area.
In general, the prior art backside-cooled catalytic reactors include a multitude of catalytic channels, where each individual catalytic channel is in essence a separate catalytic reactor. As a result, variations in fuel/air ratio from channel to channel (due to imperfect premixing, for example) can lead to different degrees of combustion and heat release in different channels. Likewise, variations in inlet temperature from channel to channel can also lead to variations in combustion behavior in different channels. Rate of reaction, catalyst light-off length, and maximum gas or surface temperature can all be affected by the temperature and fuel/air ratio at a channel inlet. In addition, manufacturing tolerances may result in unequal physical properties of different channels. Properties which may vary include channel size, wall thickness, catalyst or washcoat thickness, and catalyst loading; each of these may affect combustion behavior. In essence, multiple catalytic channels can produce widely varying degrees of catalytic combustion.
Because there is no mixing between separate catalytic channels in the prior-art backside-cooled reactors, the reactors suffer the above-mentioned disadvantages of sensitivity to premixing (fuel/air ratio) and sensitivity to inlet temperature uniformity. Given that all real systems have some level of gas-stream non-uniformity, these sensitivities translate to a narrowed operating range.
It has now been found that structures and methods that provide an un-partitioned exothermic catalytic reaction channel and multiple cooling channels offer superior performance. The un-partitioned exothermic catalytic reaction channel allows for continual mixing of the fuel/oxidant stream within the channel leading to a more uniform combustion and a wider operating range.
In addition, the structure of the present invention is more flexible, facilitating cross-stream area changes in the streamwise or longitudinal direction, since there is no constraint that walls contact each other to form multiple catalytic channels. Thus, the invention can be used to vary the bulk fluid properties in the streamwise or longitudinal direction via cross-stream area changes. In particular, it may be desirable to reduce the velocity of the fuel/air mixture after it has entered the exothermic catalytic reaction channel, to provide greater residence time for reaction within the reactor, while maintaining sufficient velocity at the reactor inlet to prevent flashback to the fuel/oxidant mixture upstream of the reactor.